The present invention relates to apparatus for compressing and decompressing a digital image signal, and, more particularly, is directed to apparatus which hierarchically encodes the digital image signal into a plurality of signals that have different resolution levels, and corresponding decoding apparatus.
A hierarchical encoding technique that forms an image signal into a hierarchy of signals of decreasing resolution has been proposed. With this encoding technique, image signals in a plurality of hierarchical levels are transmitted through one transmission path, for example, one communication path or one recording/reproducing process. In addition to the resolution levels, such hierarchical encoding technique can be applied to image reduction.
Television monitors having different resolution reproduce the transmitted and encoded signals of the different hierarchical levels. For example, standard resolution video signals, high resolution video signals for high definition television sets, computer display image data, low resolution video signals for image databases that are retrieved at high speed have been employed.
FIG. 1 shows an example of the above-described hierarchical encoding apparatus, sometimes referred to as a pyramid encoding apparatus. In this example, image signals are encoded in four hierarchical levels. In the first hierarchical level, the number of pixels of an image signal is not reduced. In the second, third and fourth hierarchical levels, the number of pixels of the image signal is reduced by 1/4, 1/16 and 1/64, respectively, relative to the number of pixels of the image signal in the first hierarchical level.
A digital image signal, referred to as an image signal in the first hierarchical level, is supplied to input terminal 1 of FIG. 1 which supplies the input image signal to thin-out filter 2 and subtractor 5.
The thin-out filter 2 selects certain pixels of the first hierarchical signal to form a second hierarchical signal, and supplies the second hierarchical signal to thin-out filter 3, subtractor 6, and interpolation filter 8.
The interpolation filter 8 interpolates the pixels of the second hierarchical signal to form an interpolated first hierarchical signal and supplies the interpolated first hierarchical signal to the subtractor 5.
The subtractor 5 subtracts, on a pixel by pixel basis, the interpolated first hierarchical signal from the original first hierarchical signal to produce a differential signal, and supplies the differential signal to an encoder 11.
The encoder 11 encodes the differential signal to produce an encoded first hierarchical level signal, and supplies the encoded first hierarchical level signal to output terminal 15.
Similarly, the thin-out filter 3 selects certain pixels of the second hierarchical signal to form a third hierarchical signal, and supplies the third hierarchical signal to thin-out filter 4, subtractor 7, and interpolation filter 9.
The interpolation filter 9 interpolates the pixels of the third hierarchical signal to form an interpolated second hierarchical signal and supplies the interpolated second hierarchical signal to the subtractor 6.
The subtractor 6 subtracts, on a pixel by pixel basis, the interpolated second hierarchical signal from the actual second hierarchical signal to produce a differential signal, and supplies the differential signal to an encoder 12.
The encoder 12 encodes the differential signal to produce an encoded second hierarchical level signal, and supplies the encoded second hierarchical level signal to output terminal 16.
The thin-out filter 4 selects certain pixels of the third hierarchical signal to form a fourth hierarchical signal, and supplies the fourth hierarchical signal to interpolation filter 10 and encoder 14.
The interpolation filter 10 interpolates the pixels of the fourth hierarchical signal to form an interpolated third hierarchical signal and supplies the interpolated third hierarchical signal to the subtractor 7.
The subtractor 7 subtracts, on a pixel by pixel basis, the interpolated third hierarchical signal from the actual third hierarchical signal to produce a differential signal, and supplies the differential signal to an encoder 13.
The encoder 13 encodes the differential signal to produce an encoded third hierarchical level signal, and supplies the encoded third hierarchical level signal to output terminal 17.
The encoder 14 encodes the fourth hierarchical signal to produce an encoded fourth hierarchical level signal, and supplies the encoded fourth hierarchical level signal to output terminal 18.
Each of the thin-out filters 2, 3, and 4 reduces the number of pixels horizontally and vertically by 1/2, that is, reduces the total number of pixels by 1/4. Thus, the number of pixels of the image signal in the second, third and fourth hierarchical levels is 1/4, 1/16 and 1/64, respectively, of the number of pixels of the image signal in the first hierarchical level.
Each of the interpolation filters 8, 9, and 10 interpolates pixels horizontally and vertically, that is, outputs an interpolation signal with pixels increased by a factor of 4 from the number of pixels of the input signal.
Each of the encoders 11, 12, and 13 compresses the data amount of the respective signal supplied thereto using adaptive quantizing techniques such as a linear quantizing technique, non-linear quantizing technique, differential pulse code modulation (DPCM), adaptive dynamic range coding (ADRC), or another compression technique.
FIG. 2 shows a hierarchical decoding apparatus corresponding to the above-described hierarchical encoding apparatus.
First, second, third, and fourth hierarchical level signals are supplied to input terminals 21, 22, 23, and 24, respectively, and thence to decoders 25, 26, 27, and 28, respectively. The decoders 25, 26, 27, 28 perform a complementary function relative to the encoders 11, 12, 13, 14, respectively.
The decoder 28 outputs a fourth hierarchical level image signal to an output terminal 38 and an interpolation filter 34. The interpolation filter 34 forms an interpolated third hierarchical level signal and supplies the interpolated third hierarchical level signal to adder 31.
The decoder 27 outputs a differential third hierarchical level signal to the adder 31, which adds the differential third hierarchical level signal and the interpolated third hierarchical level signal to form a third hierarchical level signal, and supplies the third hierarchical level signal to an output terminal 37 and an interpolation filter 33. The interpolation filter 33 forms an interpolated second hierarchical level signal and supplies the interpolated second hierarchical level signal to adder 30.
The decoder 26 outputs a differential second hierarchical level signal to the adder 30, which adds the differential second hierarchical level signal and the interpolated second hierarchical level signal to form a second hierarchical level signal, and supplies the second hierarchical level signal to an output terminal 36 and an interpolation filter 32. The interpolation filter 32 forms an interpolated first hierarchical level signal and supplies the interpolated first hierarchical level signal to adder 29.
The decoder 25 outputs a differential first hierarchical level signal to the adder 29, which adds the differential first hierarchical level signal and the interpolated first hierarchical level signal to form a first hierarchical level signal, and supplies the first hierarchical level signal to an output terminal 35.
In the above-described conventional hierarchical encoding apparatus, signals in lower hierarchical levels are interpolated to form signals in higher hierarchical levels by the interpolation filters 8, 9, and 10, and then used to form differential signals between respective original image signals and respective interpolation signals. However, the accuracy of the interpolation filters is not sufficiently high. Thus, the values of the differential signals become relatively large. After the differential signals are encoded, the amount of data to be transmitted is still too large.